The present invention generally relates to an electrochemical cell. More particularly, the present invention relates to an improved electrode structure for an electrochemical cell, particularly for an alkaline cell.
Typical alkaline electrochemical cells include a positive electrode made of manganese dioxide (MnO.sub.2), a negative electrode made of zinc, and an alkaline electrolyte made of potassium hydroxide (KOH), or the like. The positive electrode is normally formed as a hollow cylinder with its outer surface contacting the inner surface of a cell housing shaped as a can. A separator is disposed within the inside of the positive electrode to physically separate the positive electrode from the negative electrode while allowing ionic transport between the two electrodes.
The negative electrode is formed by mixing the zinc active material in the form of a zinc alloy powder with the alkaline electrolyte and a gelling agent. The mix is dispensed within the hollow middle area defined by the inner surface of the separator within the positive electrode. Subsequently, a collector assembly is inserted into the open end of the cell housing with a collector nail extending down within the negative electrode/electrolyte gel. An outer cover is then placed over the collector assembly and the cell housing walls are then crimped over the outer cover to seal the cell.
Japanese published Application No. 7-254406 discloses the use of a gelled zinc negative electrode in which a gelling agent and an alkaline electrolyte are mixed, and the negative electrode active material comprises non-amalgamated zinc powder in the shape of balls and long slender elements to increase the surface area exposed to alkali electrolyte.
In the manufacture and use of these known batteries or cells, the lowest zinc volume percent in the negative electrode that manufacturers utilize is about no less than 28 percent in the negative electrode gel in order to both match the positive electrode's rate of electrochemical output and provide sufficient particle-to-particle and particle-to-collector contact to maintain the negative electrode's electrical conductance. Below this amount, voltage instability occurs, as well as the resulting production of a cell structure having high sensitivity to shock and vibration, which cause the zinc particles to migrate away from the current collector nail thereby decreasing cell efficiency.
In order to provide the maximum electrochemical activity and a minimum of limiting polarization, it is desirable to operate a battery at as low a current density on the zinc as possible while still producing the required amount of total current from the system. Accordingly, alkaline batteries conventionally employ electrodes made from powdered active materials to obtain the highest possible surface area per unit weight or volume, and thus minimize the current density.
Conventional zinc powder is powder that has been produced by air-jet atomization of molten zinc. It consists of irregularly shaped particles, ranging from lumpy or distorted spheroids to elongated, tuberous forms. In typical battery grade zinc powder, the full population of material consists of many individual particles of a wide range of sizes and shapes. The median value of the particle size for negative electrodes, as determined by sieving, is approximately 100 to 300 microns. The extremes of particle sizes range from 20 to 1000 microns.
While zinc powder negative electrodes are relatively efficient at low discharge rates, such electrodes are much less efficient when discharged at high rates. Given that most new battery-powdered devices have high current demands, causing the batteries to discharge at high rates, there exists a strong demand for batteries having greater high-rate performance.
In International PCT Patent Publication No. WO 98/20569, entitled ZINC ANODE FOR AN ELECTROCHEMICAL CELL, by Lewis F. Urry, published on May 14, 1998, a negative electrode is disclosed that includes zinc flakes. The zinc flakes differ from the prior zinc powder particles in that the zinc flakes have a thickness many times smaller than both their length and width, for example, 10 to 20 times smaller. The disclosed flakes have a thickness on the order of 0.001 inch and lengths and widths of 0.024 to 0.04 inch. While the use of zinc flakes improves the high-rate performance of the negative electrode of an alkaline electrochemical cell, there remains room for further improving negative electrode performance particularly at high drain rates.
It has been discovered that discharge of zinc in an alkaline cell starts near the positive electrode and then proceeds away from the positive electrode. Because the reaction product (e.g., zinc oxide and zinc hydroxide) resulting from the discharge of zinc is more voluminous than the zinc itself, a reaction product skin tends to form between the positive and negative electrodes if there is not enough space to accommodate the reaction product. While such a skin still allows some electrolyte to pass through, the reacting zinc behind the skin does not receive hydroxyl ions from where they are formed in the positive electrode fast enough to offset those consumed by the reacting zinc. As a result, polarization occurs leading to premature cell failure.
In most cell designs, the current collector, which is often in the form of a nail, is located in the center of the negative electrode. Because most of the zinc discharge occurs at the outer periphery of the negative electrode near the positive electrode interface, it is necessary to maintain a continuous path of connected zinc from the reacting site to the collector nail to facilitate electron transfer. When zinc powders or flakes are used, many particles must touch to form an electron conduction path back to the collector nail. However, because the zinc powder or flakes only constitute approximately 30 percent of the negative electrode volume, any physical shocks to the cell may cause the particles to shift and lose contact. Thus, excess zinc is often added to the negative electrode only to serve as an electron conductor. The excess zinc, however, is not discharged during the life of the cell and takes up valuable space within the cell that could otherwise be used for extra electrolyte to fuel reactions or to hold discharge reaction product while still leaving space for ion transfer. Alternatively, some of the space could be used to increase the amount of MnO.sub.2 in the positive electrode.